Digital image watermarking methods are used to embed information into a digital image. In some cases, the watermark should be visible, for example if it is a copyright notice or the like. In other cases, it is desirable that the watermark is imperceptible to a human observer so that the watermarking method can be used for steganographic purposes.
Traditional digital image watermarking methods add the information to be embedded to the middle frequency coefficients of the discrete cosine transform domain in a greyscale image or to the luminance domain of a colour image.
Applications of imperceptible digital watermarking include copyright protection systems, in which a device analyses a watermark retrieved from the digital signal to be copied before deciding whether the copy should be made depending on the analysis. Another application is source tracing, in which a unique watermark is embedded into a digital signal identifying a point of origin. If a copy is found then the unique watermark can be retrieved from the copy and used to identify the point of origin.